1. Field of the Invention
This invention relates to a pneumatic pulverizer and pulverizing method by use of a jet gas stream (high pressure gas). Particularly, the present invention pertains to a pneumatic pulverizer and pulverizing method for forming toner or colorant resin powder with an toner to be used for image forming method such as electrophotography with good efficiency.
2. Related Background Art
Toner or colorant resin powder for toner to be used for an image forming method such as electrophotography generally contains at least a binder resin and a colorant or magnetic powder. Toner develops the electrostatic image formed on a latent image carrier, and the toner image formed is transferred onto a transfer material such as plain paper or plastic film. Then the toner image on the transfer material is fixed by a fixing device such as a heating fixing means, a pressure roller fixing means or a heating pressurization roller fixing means. Accordingly, the binder resin to be used in toner has the characteristic of plastic deformation when heat and/or pressure is applied.
Presently, toner or colorant resin powder for toner is prepared by melting and kneading a mixture containing at least a binder resin and a colorant or magnetic powder (further containing, if desired, a third component), cooling the melted and kneaded mixture, pulverizing the cooled product and classifying the pulverized product. Pulverization of the cooled product is practiced generally by coarsely (or moderately) pulverizing the product by means of a mechanical impact mill and then subsequently finely pulverizing the pulverized coarse powder by means of a pneumatic pulverizer using a jet gas stream.
Pneumatic pulverizer by use of a jet gas stream conveys the starting powder with a jet gas stream and permits the starting powder to impinge on an impinging member, thereby pulverizing the powder through its impact force.
In the prior art, the impinging surface 14 of the impinging member used in such a pulverizer had a planar shape vertical to or slanted at an angle of, for example, 45.degree. relative to the jet gas stream direction (the axial direction of the accelerating pipe) carrying the starting powder, as shown in FIG. 5, FIG. 6 and FIG. 8 (see Japanese Laid-open Patent Publications Nos. 57-50554 and 58-143853).
In the pulverizer shown in FIG. 5, the starting powder having a coarse particle size is fed from the throwing inlet 1 into the accelerating pipe 3, and the starting powder is thrashed with the jet gas stream blown out from the jet nozzle 2 against the impinging surface 14 of the impinging member 4 to be pulverized through its impact force, and discharged out of the pulverizing chamber from the discharging outlet 5. However, when the impinging surface 14 is vertical to the axial direction of the accelerating pipe 3, the starting powder blown out from the jet nozzle 2 and the powder reflected against the impinging surface 14 will coexist at a higher rate, whereby the powder concentration in the vicinity of the impinging surface 14 becomes higher to give poor pulverization efficiency. Further, pulverization is effected mainly due to primary impingement on the impinging surface 14, and therefore it cannot be said that secondary impingement on the pulverizing chamber wall 6 is effectively utilized. Further, in a pulverizer wherein the angle of the impinging surface is vertical to the accelerating pipe 3, fusion and aggleomerated product are liable to be generated due to local heat generation during pulverization of a thermoplastic resin, whereby stable running of the device is difficult and causes a reduction in pulverizing ability. For this reason, it has been difficult to use powder at high concentration.
In the pulverizer shown in FIG. 6, in which the impinging surface 14 is slanted relative to the axial direction of the accelerating pipe 3, the powder concentration in the vicinity of the impinging surface 14 becomes lower as compared with the pulverizer shown in FIG. 5, but the pulverization pressure is dispersed and lowered. Further, it cannot be said that the secondary impingement on the pulverizing chamber wall 6 is effectively utilized.
As shown in FIG. 6 and FIG. 7, in a device having an impinging surface 14 slanted at an angle of 45.degree. relative to the accelerating pipe, there is little problem as mentioned above during pulverization of a thermoplastic resin. However, because the impact force used for pulverizing during impingement is small, and further pulverization through secondary impingement on the pulverizing chamber wall 6 is little, the pulverizing ability will drop to 1/2 to 1/1.5 as compared with that of the pulverizer shown in FIG. 4.
The pulverizer shown in FIG. 8, wherein the impinging surface 14 is slanted downward relative to the axial direction of the accelerating pipe 3, the powder concentration in the vicinity of the impinging surface 14 becomes lower as compared with the pulverizer shown in FIG. 5. Further, although secondary impingement on the pulverizing chamber wall 6 is effectively utilized, as shown in FIG. 9, only the lower wall surface is substantially utilized in secondary impingement on the pulverizing chamber wall 6. Accordingly, it would be desirable to have a pulverizer and a pulverizing method having much better pulverization efficiency.